More effective monitoring of trace organic compounds such as pesticides and environmental toxins has become increasingly important. Examples of sample matrices that are monitored include soil, oil, water, fruits, vegetables, foams, and carpets.
Quite often, an untreated sample cannot be tested directly in an analytical instrument. Typically, a sample extraction process is performed first, which is a process for separating the compound of interest (i.e. analyte) from the sample matrix. A sample extraction can include treating the sample with an organic solvent to preferentially dissolve the analyte and separate it from other components of the sample. The organic solvent containing the dissolved analyte can then be tested with an analytical instrument. In other instances, the organic solvent may be removed or at least partially removed to concentrate the analyte before performing the analytical test. Examples of analytical instruments suitable for analyzing an extracted sample are gas chromatography and liquid chromatography with suitable detectors such as a mass spectrometer, ultraviolet-visible spectrometer, fluorimeter, flame ionization detector, electrochemical detector, and the like
A particular type of solvent extraction is accelerated solvent extraction (ASE). This technique is performed using an organic solvent at an elevated temperature and pressure. The elevated pressure increases the solvent boiling point thereby allowing a liquid extraction to be conducted at higher solvent temperatures. In general, a higher temperature will speed up analyte dissolution and the extraction process.
Many trace organic compounds of interest have the property of being volatile or semi-volatile. A challenge in monitoring such compounds is that many types of sample matrices contain water or residual moisture. Drying the sample in an oven is not a viable option because the volatile and semi-volatile compounds can also be removed from the sample. The presence of water can interfere with the extraction process efficiency. For example, the presence of an aqueous phase with dissolved analyte will decrease the extraction efficiency into the organic solvent. If solvent evaporation is needed, the analyte can preferentially concentrate into the aqueous phase and make the analysis more difficult.
The presence of water can also interfere with the analytical method. Analytical techniques such as gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS) can be sensitive to moisture depending on the type of stationary phase materials.
One approach for removing residual water from an organic phase involves the addition of an insoluble inorganic salt that sorbs water. This technique is also referred to as “salting out” in the literature. When the analyte tends to partition between the aqueous phase and the organic phase, recovery in the organic phase decreases. To counter this effect, an inorganic salt that has a high affinity for the aqueous phase is added. The salt sorbs water and forms a hydrate, which causes the analyte to shift from the aqueous phase to the organic phase. Thus, the addition of salt causes the analyte to preferentially dissolve into the organic phase. Salts such as sodium sulfate, calcium chloride, magnesium sulfate, and calcium sulfate may be used for the purpose of moisture removal. The organic solvent phase is removed after the salt exposure by filtration or by decanting. The process for removing the salt by filtration can be difficult because it tends to clump together when water is present. The inorganic salt is typically added in increments so as to prevent an excess amount from being added. Filters can be easily clogged or take a large amount of time to filter the organic solvent making the process difficult to automate. Applicants believe that there is a need to develop a drying agent that can efficiently and effectively sorb water and be easily separated from the organic solvent such that the process can be automated.
Instead of salts, polymeric drying agents can also be used to remove water from a sample. An example of such a polymeric drying agent is the sodium salt of polyacrylic acid. Although this polymeric drying agent removes water by absorbing it into the polymer matrix, the water absorbing capacity decreases as the ionic strength increases. Since most sample ionic strengths are not known a priori, it would be difficult to use this polymeric drying agent particularly with unknown samples that may have a high ionic strength. Another limitation of polyacrylic acid is that it absorbs water poorly under higher temperature conditions.
Applicants believe that there is a need to develop a drying agent that can efficiently and effectively sorb water at both elevated temperature and pressure so that it can be used with accelerated solvent extraction and other extraction techniques such as supercritical fluid extraction, microwave extraction, Soxhlet extraction and the like. In addition, applicants believe that there is a need for the drying agent to efficiently and effectively sorb water for a sample containing a wide range of salt concentrations. This would allow the drying agent to be used with a wide range of sample types.